In an attempt to understand the genetic control and moleculr mechanisms of sister chromatid exchange in the yeast Saccharomyces cerevisiae, forty-three putative mutants with elevated levels of sister chromatid exchange have been isolated. These mutants we isolated in a strain that is deleted for the resident HIS3 gene and that contains two truncated copies of the HIS3 genes integrated near the centromere of chromosome IV; unequal sister chromatid exchange between homologous regions of the HIS3 gene can restore the functional gene. Mutants elevated for sister chromatid exchange wee identified based on their increased ability to form HIS+ prototrophs compared to the control unmutagenized clones. Experiments are underway to characterize these mutants in terms of general effects on recombination, DNA damage induced events, mutagenesis and molecular/enzymological defects.